The present invention relates to a call processing message converter and message converting method in an IP-PBX system, capable of controlling IP terminals equally to legacy terminals by transceiving user information with IP terminals. The message converter works best especially for the IP-PBX system, wherein the system includes: a hub connected to an Internet protocol (IP) network, having a plurality of LAN communication ports; user information interface connected to the LAN communication port of the hub for generating user information and for outputting the generated user information or for converting received exchange user information to an identifiable state to users; at least one IP terminal for performing VoIP function in conforming to a pre-designated data communication protocol procedure; IP driver connected to the LAN communication port of the hub for controlling Internet protocol endpoints; and IP-PBX, mounted with a database where telephone numbers of a legacy terminal and the plurality of IP terminals and corresponding terminal information, and an exchange interface and a gateway connected to the LAN communication port of the hub, respectively, for conducting call processing services corresponding to user information that is sent to the exchange interface and call connection information that is generated from the legacy terminal. The call processing message converter is built in the IP driver, and the converter analyzes an ARQ (Admission request) message transported from a terminating IP terminal, transports port information and digit information of the IP terminal to a call processing module inside of the IP-PBX, and transports a termination seizure message transported from the call processing module and user control information of a terminating IP terminal to a terminating terminal.